Scientists use ice cores as well as many other sources of information (geological, botanical, etc.) to reconstruct the history of past climate variations and glaciations. When all this information is pieced together, this is the story it tells:
About 20,000 years ago, the southwestern Yukon region, like all of the Northern Hemisphere, was in the grip of the last ice age. The St-Elias Mountains and most of the southern Yukon were then covered by the Cordilleran Ice Sheet, which at times reached out to meet the western edge of the Laurentide Ice Sheet, itself stretching westward from what is now the Keewatin region of the Northwest Territories. Meanwhile, large parts of central and northern Yukon remained ice-free. This was the time when lowered sea levels linked Alaska to Asia, allowing the ancestors of present-day First Nations to emigrate to North America. (see Beringia)
Between 12,000 and 10,000 years ago, the Earth's "wobble" about its axis of rotation caused the amount of summer sunlight reaching northern latitudes to increase considerably, signaling the end of the ice age. The Earth then entered the present, warmer interglacial period, called the Holocene. As the large ice sheets melted away, glaciers in the southwestern Yukon retreated back to the cold, high plateaus and valleys of the St. Elias Mountains.
However by the middle of the Holocene the climate of the Northern Hemisphere had begun to cool down again, as Earth's axis once more tilted away from the Sun. The decreasing summer insolation brought a period of renewed ice expansion, or "Neoglaciation", that began ~3500 years ago, and saw glaciers in the Yukon, British Columbia and Alaska advance far beyond their early Holocene limits. The Neoglacial glacier advances in the St. Elias Mountains were probably driven by a combination of wetter winters and cooler summers. This phase of expansion culminated during the so-called Little Ice Age, a period of generally cold, wet and stormy weather that was experienced by much of the Northern Hemisphere between the 14th and mid- to late 19th centuries. By then the terminus of the Kaskawulsh glacier (for example) was probably ~25 km further down-valley than it had been during most of the Holocene. The Little Ice Age was not unique, however, and is just the latest in a series of century-long climate swings that have punctuated the Holocene.
Most records of climate show that the Little Ice Age ended ~150 years ago, and since then many Northern Hemisphere glaciers have been retreating. However repeated measurements of polar and alpine glaciers world-wide beginning in the 20th century and recent;y improved by satellite imagery show that the rates of glacier retreat, thinning and melting have accelerated over the last ~20 to 30 years. One recent study, for example, reveals that many large glaciers in southeast Alaska and the southwestern Yukon region have experienced increased thinning rates between ~1995 and 2001 relative to the ~1950 to 1995 period.
During the same period polar regions of the globe have experienced rapid climatic warming, and it is now believed my most climate scientists that the accelerating meltdown is partly to blame on human influence on the Earth's climate related to greenhouse gas buildup and land use changes. While these changes are unlikely to impact Mount Logan's icy plateau anytime soon, they could lead to major changes in the large glaciers below and the lakes and rivers that they feed.